The studies reported here center around two major findings. The first is that the non-invariant NKT cells previously identified in UC are in fact capable of responding to a self-glycolipid, a member of the sulfatide family of NKT cell stimulants. This finding strongly suggests that NKT cells in UC are responding to a self antigen and that UC qualifies as a classical autoimmune disease. The second is that NKT cells in UC bear the IL-13R&#945;2 receptor that: 1) is up-regulated by sulfatide stimulation;and 2) is the receptor through which IL-13 interacts with NKT cells to induce enhancement of NKT cell cytotoxicity in an autocrine fashion. This receptor is therefore critical to NKT cell-mediated pathologic effects. The response of UC lamina propria NKT cells to sulfatide was shown in three ways: it induced NKT cell cytokine production, induced NKT cell cytotoxic function and augmented NKT cell expression of the IL-13R&#945;2 receptor, but not the IL-13R&#945;1 receptor. These findings were consistent with and help explain our previous observation that NKT cells in UC respond to stimulation by a B cell stably expressing high levels of CD1d in the absence of exogenous antigen, since in the latter case it is possible that the CD1d on the B cell was loaded with an endogenous self-glycolipid that cross-react with sulfatide. The responses elicited by the lyso-sulfatide are also consistent with our previous studies in that they showed that NKT cells in UC can be stimulated by the aforementioned transfected B cell to mediate an atypical Th2 response characterized by production of IL-13 but not IL-4 (or various Th1 cytokines) and to exhibit cytotoxicity for epithelial cells ( ). A totally new observation obtained from the present study was that, as eluded to above, sulfatide upregulation of the IL-13R&#945;2 receptor not only results in enhancement of the number of cells bearing the receptor, but perhaps more importantly to a great increase in the intensity of receptor expression on individual cells. This observation implies that stimulation of NKT cells by sulfatide leads to a positive feedback circuit in which induction of cytotoxicity by TCR-recognition of antigen can ultimately be enhanced by increased expression of a receptor that also mediates cytotoxity. The observation that sulfatide stimulation of NKT cells results in upregulation of the IL-13R&#945;2 but not the IL-13R&#945;1 prompted us to determine if this receptor could server as a surrogate marker of NKT cells in UC. To investigate this possibility we turned to the oxazolone-colitis model, a colitis driven by IL-13 and NKT cells, and showed that administration of IL-13 linked to pseudomonas exotoxin (IL-13-PE) ablates oxazolone-colitis by targeting and eliminating NKT cells and its IL-13 secretion. Since it is well documented that the cytotoxic effect of IL-13-PE depends on binding to the IL-13R&#945;2 receptor ( ), this study provided strong evidence that NKT cells involved in a UC type inflammation bear this receptor. Additional studies showing that invariant TCR-bearing cells detected with &#945;-GalCer-tetramer also bear IL-13R&#945;2 supported this conclusion. In parallel studies of humans with UC we first showed that circulating CD4+ T cells in patients have easily detectable cells bearing the IL-13R&#945;2 whereas patients with CD have barely detectable levels. Furthermore, depletion of cells with IL-13-PE led to co-depletion of receptor-bearing cells and IL-13-producing cells. These findings relating to IL-13R&#945;2-bearing cells in the circulation of UC patients presaged findings in the inflamed tissues of patients. In the latter case, we found that the majority of lesional lamina propria mononuclear cells (approximately 70%) bear IL-13R&#945;2 whereas the percentage of cells bearing the IL-13R&#945;1 receptor was low. In addition, the receptor-bearing cells also bear the NKT cell marker, CD161 and most (if not all) are IL-13-producing cells. Finally, we showed that IL-13-PE depletion of UC lamina propria cells led to greatly reduced IL-13 production and cytotoxic activity, as in the case of circulating cells, again validating the concept that IL-13R&#945;2 is an NKT cell marker in UC. These studies provide unequivocal evidence that IL-13-producing cells bearing NKT cell markers are more than a minor sub-population of cells in actively inflamed UC mucosa there are no proper lesions in UC as the disease is diffuse;on the contrary, they make up the majority of cells in the inflamed areas. This fact greatly increases the likelihood that UC is primarily due to these cells and the IL-13 they produce.